In the conventional method of manufacturing fluorocarbons of the formula CHCl.sub.n F.sub.3-n, where n is 1 or 2, the fluorocarbon stream exiting from the reactor generally contains a substantial quantity of acidic impurity, such as HCl. Removal of this acidic impurity from the fluorocarbon is usually accomplished by absorption. However, regardless of the type of absorber used, the fluorocarbon stream exiting from the absorber will still contain a harmful quantity of residual HCl impurity, usually in the range of about 0.10-0.60% by weight. This acidic impurity due to its highly corrosive nature, unless removed from the fluorocarbon, will seriously interfere with the further use and processing of the fluorocarbon.
In the past, this residual acidic impurity has been removed from the fluorocarbon stream by scrubbing with an aqueous solution of a base of intermediate strength, generally with a base having a pH in aqueous solution below about 13. In conventional scrubbing systems, an aqueous solution of trisodium phosphate has been generally employed. This basic compound has a pH of about 13 in aqueous medium. Stronger bases, such as sodium hydroxide, could not be used as sodium hydroxide readily hydrolyzes the fluorocarbons to formic acid and CO.sub.2. Consequently, use of strong bases (pH &gt; 13) is avoided in the purification of acid-contaminated fluorocarbons of the formula referred to above.
The use of trisodium phosphate (Na.sub.3 PO.sub.4) for scrubbing purposes has been found quite satisfactory. Nonetheless, its utilization has certain disadvantages apart from cost considerations. Thus, during the neutralization of HCl with an aqueous trisodium phosphate solution, as the pH of the solution decreases, generation of acidic phosphate salts occur, which reduce the effectiveness of the trisodium phosphate solution as a scrubbing medium. The reaction between the HCl contaminant of the fluorocarbon stream and the trisodium phosphate neutralizing agent is shown below. EQU Na.sub.3 PO.sub.4 + HCl .fwdarw. Na.sub.2 HPO.sub.4 + NaCl (1) EQU Na.sub.2 HPO.sub.4 + HCl .fwdarw. NaH.sub.2 PO.sub.4 + NaCl (2)
These reactions cause a significant reduction in the pH of the scrubbing medium with corresponding loss in HCl conversion efficiency. In order to restore the efficiency of HCl removal from the fluorocarbon stream in the conventional practice, the exhausted scrubbing solution is discarded and entirely replaced with a fresh solution.
Regeneration of the exhausted scrubbing solution, i.e., restoration of its trisodium phosphate content, has been found impractical. On one hand, since the exhausted solution contains a significant quantity of sodium chloride, regeneration will not remove this salt and its buildup in the regenerated scrubbing medium continues until deposition of precipitated NaCl will seriously affect the scrubbing operation and operation of the equipment associated with the scrubbing. On the other hand, the volume of scrubbing medium is generally quite large, thus regeneration of these large volumes involves extensive and expensive equipment and operating steps.
Direct disposition of the exhausted scrubbing medium into natural receiving bodies, due to its high PO.sub.4.sup.3- -ion concentration, results in environmental problems. Removal of the phosphate content of this solution prior to disposal is impractical, expensive and technically unfeasible. Thus, the above-referred to problems arising with the utilization of trisodium phosphate as scrubbing medium for fluorocarbons pose such problems which render the use of this material unsatisfactory.
It has now been discovered that the difficulties associated with the employment of trisodium phosphate as scrubbing medium for HCl contaminated fluorocarbon streams can be substantially eliminated by employing a scrubbing solution containing in combination with trisodium phosphate a controlled quantity of sodium hydroxide. Utilization of sodium hydroxide in the scrubbing medium allows in situ regeneration of acidic phosphate salt species to trisodium phosphate, thus retaining the efficiency of the scrubbing medium. At least periodic purging of a portion of the scrubbing medium and at least periodic replacement of the purged portion with fresh trisodium phosphate-sodium hydroxide solution allows control both of the sodium chloride content of the scrubbing solution and also of its pH. Additionally, purging only removes a small portion of the scrubbing medium and due to the significantly lower total phosphate ion content in this portion, disposal of the purge stream, even into natural receiving bodies, meets environmental restrictions.